Why does Mr. Tate claim Bob Ewell killed himself even though that isn't what really happened?

Atticus is convinced that his son, stabbed Ewell with a knife, making him responsible for his murder. Sheriff Tate explains to Atticus how Bob Ewell fell on his own knife and died. Atticus does not accept Tate's explanation of Ewell's death. Sheriff Tate sticks with what he said about dwell falling on his own.